Memory cell with a Frohmann-Bentchkowsky EPROM memory transistor that reduces the voltage across an unprogrammed memory transistor during a read

ABSTRACT

The unintentional programming of a memory cell having a Frohmann-Bentchkowsky p-channel memory transistor and an n-channel MOS access transistor, which can occur during a read operation when high read voltages are used, is eliminated by forming a diode-connected MOS transistor in parallel with the access transistor. The diode-connected transistor is formed to be off when an unprogrammed memory transistor is read, and to be on when a programmed memory transistor is read.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 09/082,145 for MEMORY DEVICE THAT UTILIZES SINGLE-POLY EPROM CELLS WITH CMOS COMPATIBLE PROGRAMMING VOLTAGES filed on May 20, 1998 by Alexander Kalnitsky et al., which is a continuation-in-part of application Ser. No. 09/053,309 for SINGLE-POLY EPROM CELL WITH CMOS COMPATIBLE PROGRAMMING VOLTAGES filed on Apr. 1, 1998 by Alexander Kalnitsky et al.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to Frohmann-Bentchkowsky electrically-programmable read-only-memory (EPROM) cells and, more particularly, to Frohmann-Bentchkowsky EPROM cells that reduce the voltage across an unprogrammed memory transistor during a read.

2. Description of the Related Art

In the early 1970s, an electrically-programmable read-only-memory (EPROM) transistor based on a p-channel MOS device with a completely isolated gate was introduced by D. Frohmann-Bentchkowsky (see "A Fully Decoded 2048-bit Electrically Programmable MOS-ROM", IEEE ISSCC Digest of Technical Papers, p.80, 1971).

FIG. 1 shows a cross-sectional view that illustrates a Frohmann-Bentchkowsky memory transistor 10. As shown in FIG. 1, memory transistor 10 includes spaced-apart p-type source and drain regions 16 and 18, respectively, which are formed in an n-type well 14 which, in turn, is formed in a p-type substrate 12. (Source and drain regions 16 and 18 may alternately be formed in an n-type substrate).

In addition, transistor 10 also includes a channel region 20 which is defined between source and drain regions 16 and 18, and a layer of gate oxide 22 which is formed over channel region 20. Transistor 10 further includes a gate 24 which is formed over gate oxide layer 22, and a layer of insulation material 26 which, along with gate oxide layer 22, completely encapsulates gate 24. Since gate 24 is completely isolated, it is commonly referred to as a floating gate.

In operation, transistor 10 is programmed by applying biasing voltages to well 14 and drain 18 which are sufficient to induce avalanche breakdown. For example, avalanche breakdown is induced by applying ground to well 14 and a negative breakdown voltage to drain region 18 (while either grounding or floating source region 16), or by applying a positive breakdown voltage to well 14 and ground to drain region 18 (while floating or applying the positive breakdown voltage to source region 16).

The biasing voltages which are sufficient to induce avalanche breakdown establish a strong electric field across the drain-to-well junction depletion region. The strong junction electric field accelerates electrons in the junction depletion region (which are formed from thermally-generated electron-hole pairs) into hot electrons which then have ionizing collisions with the lattice, thereby forming "substrate hot electrons".

A number of these substrate hot electrons penetrate gate oxide layer 22 and begin accumulating on floating gate 24 due to the relatively positive potential on floating gate 24 with respect to drain region 18.

The potential on floating gate 24 is defined by the voltages which are coupled to floating gate 24 from well 14, source region 16, and drain region 18, and the surface area of floating gate 24 that is formed over these regions. Thus, since floating gate 24 is only marginally formed over source and drain regions 16 and 18, the potential on floating gate 24 is primarily determined by the voltage applied to well 14.

Therefore, when ground is applied to well 14 (and source region 16), and the negative breakdown voltage is applied to drain region 18 during programming, the potential on floating gate 24 is slightly less than ground which, in turn, is relatively positive with respect to the negative breakdown voltage applied to drain region 18.

(If a positive breakdown voltage is applied to well 14 and ground is applied to drain region 18, then floating gate 24 will have a potential slightly less than the positive breakdown voltage which, in turn, is positive with respect to ground which is applied to drain region 18).

Transistor 10 is read by applying ground to well 14 and source region 16, and a read voltage to drain region 18. (Alternately, transistor 10 can also be read by applying a first voltage (Vdd) to n-well 14 and source region 16, and a smaller second voltage (<Vdd) to drain region 18.)

If transistor 10 has been programmed, the negative charge on floating gate 24 causes channel region 20 to invert (which inversion is permanent as long as the negative charge remains on floating gate 24). As a result, the voltages applied to source and drain regions 16 and 18 cause a current to flow from drain region 18 to source region 16.

On the other hand, if transistor 10 has not been programmed, the slight negative charge which is coupled to floating gate 24 is insufficient to invert channel region 20. As a result, channel region 20 remains in accumulation. Thus, when the voltages are applied to source and drain regions 16 and 18, no current is able to flow.

Memory transistor 10 is erased by irradiating transistor 10 with ultraviolet (UV) light to remove the electrons. The UV light increases the energy of the electrons which, in turn, allows the electrons to penetrate the surrounding layers of oxide.

Thus, the Frohmann-Bentchkowsky transistor utilizes a p-channel MOS-type device to inject electrons onto the floating gate (as contrasted with more recent EPROM devices that use an n-channel MOS-type device to inject electrons onto the floating gate).

Although some of the first EPROMs sold by Intel™ were based on the Frohmann-Bentchkowsky transistor, the transistor was soon replaced with alternate structures, and has since fallen into relative obscurity.

The grandparent invention disclosed that by forming a Frohmann-Bentchkowsky memory transistor to have a physical gate length which is less than the physical gate length of a CMOS transistor, as defined by a predetermined design rule, the Frohmann-Bentchkowsky memory transistor is programmable with the voltages that are allowed under the predetermined design rule.

The parent invention disclosed an array structure that can be used with multiple Frohmann-Bentchkowsky memory transistors, along with a method for programming and reading the transistors in the array.

SUMMARY OF THE INVENTION

A memory cell in accordance with the present invention utilizes a Frohmann-Bentchkowsky memory transistor, an access transistor, and a diode-connected transistor to limit the read voltage that is dropped across the memory transistor when the cell is read. By limiting the voltage which is dropped across the cell when the cell is read, unintentional programming of the cell can be eliminated.

The memory cell of the present invention includes a memory transistor of a first conductivity type. The memory transistor has a source, a drain, and a floating gate. The floating gate, in turn, has a physical gate length.

The memory cell also includes a first transistor of a second conductivity type. The first transistor has a source, a drain connected to the drain of the memory transistor, and a gate. The gate, in turn, has a physical gate length which is defined by a design rule, and which is greater than the physical gate length of the memory transistor.

The memory cell further includes a non-linear circuit element which is connected to the drains of the first transistor and the memory transistor. The non-linear circuit element includes a second transistor of the second conductivity type. The second transistor has a source, a drain, and a gate connected to the drains of the first transistor and the memory transistor. The gate of the second transistor has a physical gate length which is greater than the physical gate length of the memory transistor.

A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a Frohmann-Bentchkowsky EPROM memory transistor 10.

FIG. 2 is a cross-sectional view illustrating a CMOS logic circuit 100 in accordance with the grandparent invention.

FIG. 3 is a graph illustrating a series of programming voltages versus physical floating gate lengths in accordance with the grandparent invention.

FIG. 4 is a schematic diagram illustrating a portion of a memory device 200 in accordance with the parent invention.

FIG. 5 is a plan view illustrating a layout 300 of a portion of device 200 in accordance with the parent invention.

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 5.

FIG. 7 is a schematic diagram illustrating a portion of a device 400 in accordance with a first alternate embodiment of the parent invention.

FIG. 8 is a plan view illustrating a layout 500 of a portion of device 400 in accordance with the parent invention.

FIG. 9 is a cross-sectional view taken along line 9--9 of FIG. 8.

FIG. 10 is a schematic diagram illustrating a portion of a device 1000 in accordance with a second alternate embodiment of the parent invention.

FIGS. 11A-11E are timing diagrams illustrating the operation of device 1000.

FIGS. 12A-12E are timing diagrams illustrating the operation of device 1000.

FIG. 13 is a schematic diagram illustrating a portion of a device 1300 in accordance with a third alternate embodiment of the parent invention.

FIG. 14 is a schematic diagram illustrating a Frohmann-Bentchkowsky EPROM memory cell 1400 in accordance with the present invention.

FIG. 15 is a schematic diagram illustrating a portion of a memory cell 1500 in accordance with the present invention.

FIG. 16 is a schematic diagram illustrating a portion of a memory device 1600 in accordance with the present invention.

FIG. 17 is a schematic diagram illustrating a portion of a memory device 1700 in accordance with the present invention.

FIG. 18 is a schematic diagram illustrating a memory cell 1800 in accordance with the present invention.

FIG. 19 is a schematic diagram illustrating a memory cell 1900 in accordance with the present invention.

FIG. 20 is a schematic diagram illustrating a memory cell 2000 in accordance with the present invention.

FIG. 21 is a schematic diagram illustrating a memory circuit 2100 in accordance with the present invention.

DETAILED DESCRIPTION

In accordance with the grandparent invention, the Frohmann-Bentchkowsky EPROM transistor of FIG. 1 is programmed by applying biasing voltages to well 14, source region 16, and drain region 18 which are sufficient to induce holes to flow from source region 16 to drain region 18 with enough kinetic energy to have ionizing collisions with the lattice, and insufficient to induce avalanche breakdown at the drain-to-well junction.

Holes will flow from source region 16 to drain region 18 when the depletion region associated with the reverse-biased drain-to-well junction extends over and overlaps with the depletion region associated with the source-to-well junction.

When the drain and source depletion regions overlap, which is known as punchthrough, the drain-to-source electric field reduces the potential energy barrier at the source-to-well junction. The reduced potential energy barrier, in turn, allows the holes in source region 16 to overcome the barrier, thereby producing a hole flow below the surface of the device from source region 16 to drain region 18.

When the holes flowing to drain region 18 are hot, the holes have ionizing collisions with the lattice that form substrate hot electrons. Some of the substrate hot electrons, in turn, penetrate the layer of gate oxide 22 and accumulate on floating gate 24. Since most of the substrate hot electrons are formed directly under floating gate 24 rather than at the drain-to-well junction, the programming of the grandparent invention is more efficient that the avalanche-breakdown programming of the prior art.

Whether the depletion region associated with the reverse-biased drain-to-well junction extends over and overlaps with the source-to-well depletion region, and whether the holes become "hot" as they flow to drain region 18, i.e., whether the holes have enough kinetic energy to have ionizing collisions with the lattice, is a function of the well doping concentration, the effective channel length, and the voltages applied to well 14, source region 16, and drain region 18.

Conventionally, semiconductor wells typically have an n-type dopant concentration in the range of 1×10¹⁶ to 1×10¹⁸ atoms/cm³ for devices based on a 0.35 micron design rule, and 1×10¹⁶ to 1×10¹⁹ atoms/cm³ for devices based on a 0.25 micron design rule, where the highest concentrations are usually found close to the silicon-oxide interface.

(As noted above, source and drain regions 16 and 18 may alternately be formed in a substrate rather than a well. Semiconductor substrates typically have an n-type dopant concentration in the range of 1×10¹⁶ a to 1×10¹⁸ atoms/cm³ for devices based on a 0.35 micron design rule, and 1×10¹⁶ to 1×10¹⁹ atoms/cm³ for devices based on a 0.25 micron design rule, where the highest concentrations are usually found close to the silicon-oxide interface.)

Increasing the dopant concentration above these conventional levels reduces the strength of the drain-to-source electric field, and the breakdown voltage of the drain-to-substrate junction. Thus, transistor 10 preferably utilizes conventional substrate and well doping concentrations.

Conventional source and drain regions, in turn, typically have a p-type dopant concentration in the range of 1×10¹⁹ to 1×10²¹ atoms/cm³ for devices based on a 0.35 micron design rule, and 1×10¹⁹ to 1×10²¹ atoms/cm³ for devices based on a 0.25 micron design rule. As a result, for a conventionally-doped p-type drain region formed in a conventionally doped n-type substrate, the typical drain-to-well breakdown voltage is approximately 6-12 volts, depending on the specifics of the technology that is used.

With respect to the effective channel length, the channel length is defined by the physical gate length, while the effective channel length is defined by the amount of lateral diffusion that occurs after the source and drain regions have been formed.

For a given fabrication process, the amount of lateral diffusion, which is relatively minor, is approximately constant. Thus, although the effective channel length is less than the physical gate length, the effective channel length is defined by the physical gate length.

In the grandparent invention, memory transistor 10 is formed to have a physical gate length which allows hot punchthrough holes to flow from source region 16 to drain region 18 when the drain-to-source voltage is less than the drain-to-well breakdown voltage.

As a general rule, shorter channel devices, e.g., 0.25 and 0.35 micron devices, use higher substrate doping densities and shallower junctions than longer channel devices, thereby making the source and drain junctions more abrupt. These abrupt junctions reduce the junction breakdown voltages, while a higher channel doping increases the voltage required to induce punchthrough. The decrease in gate length does not affect the junction breakdown, but greatly reduces the voltage required to induce punchthrough.

Thus, with respect to the biasing voltages, ground is applied to drain 18, while a positive programming voltage is applied to well 14 and source 16 which is sufficient to induce holes to flow from source region 16 to drain region 18 with enough kinetic energy to have ionizing collisions with the lattice, and insufficient to induce avalanche breakdown at the drain-to-well junction.

The exact programming voltage depends on the source and drain region diffusion structures that are used. Halo implants, for example, increase punchthrough resistance and, thereby, increase the required programming voltages.

Thus, with a physical gate length of 0.35 microns based on a 0.35 micron design rule or technology (and conventional doping concentrations and structures for the well, source, and drain), holes flowing from source region 16 to drain region 18 become hot when the voltage applied to well 14 and source region 16 is equal to approximately 5.75 volts.

Similarly, with a physical gate length of 0.25 microns based on a 0.25 micron design rule or technology (and conventional doping concentrations and structures for the well, source, and drain), holes flowing from source region 16 to drain region 18 become hot when the voltage applied to well 14 and source region 16 is equal to approximately 4.5 volts.

In addition to the above, ground may alternately be applied to well 14 and source 16 while a negative programming voltage is applied to drain 18 which is sufficient to induce holes to flow from source region 16 to drain region 18 with enough kinetic energy to have ionizing collisions with the lattice, and insufficient to induce avalanche breakdown at the drain-to-well junction.

Further, the voltages applied to well 14 and source region 16 need not be the same. The voltage applied to source region 16 may be less than the voltage applied to well 14, thereby reverse biasing the source-to-well junction, or greater than the voltage applied to well 14, thereby forward biasing the source-to-well junction. A forward bias of not more than approximately 0.7 volts should increase hole injection into the depletion region, and may increase programming efficiency.

Thus, the grandparent invention utilizes hot punchthrough holes, which are generated at voltages which are significantly less than the voltages required to breakdown the drain-to-well junction, to inject electrons onto the floating gate.

In further accordance with the grandparent invention, memory transistor 10 is integratable into a conventional CMOS logic circuit by using a physical gate length which is less than the physical gate length of the MOS transistors in the CMOS circuit (with current generation CMOS processes, the NMOS and PMOS physical gate lengths are typically the same).

FIG. 2 shows a cross-sectional view that illustrates a CMOS logic circuit 100 in accordance with the grandparent invention. As shown in FIG. 2, circuit 100 includes a PMOS transistor 120 and a Frohmann-Bentchkowsky EPROM transistor 140 which are formed in an n-well 114, and an NMOS transistor 160 which is formed in a p-type well 116. In addition, n-well 114 and p-well 116 are formed in a p-type substrate 112.

(Although circuit 100 is described as being formed in a twin-well that includes n-well 114 and p-well 116, circuit 100 may alternately be formed in a single well where transistors 120 and 140, or transistor 160, is formed directly in the substrate).

As further shown in FIG. 2, transistors 120 and 140 each have spaced-apart p-type source and drain regions 122/124 and 142/144, respectively, which are formed in n-well 114, while transistor 160 has spaced-apart n-type source and drain regions 162 and 164 which are formed in p-well 116. (The field oxide region FOX formed between regions 124 and 142 may be omitted if the regions share the same trace.)

(P-type source and drain regions 122/124 and 142/144 are shown in FIG. 2 as including PLDD regions 122'/124' and 142'/144', while N-type source and drain regions 162 and 164 are shown as including NLDD regions 162' and 164'. Alternately, halo-type LDD structures may also be used.)

In addition, transistors 120, 140, and 160 each have a channel region 126, 146, and 166, respectively, which is defined between source and drain regions 122/124, 142/144, and 162/164, respectively.

Further, transistors 120, 140, and 160 each have a layer of gate oxide 128, 148, and 168, respectively, which is formed over channel region 126, 146, and 166, respectively, and a gate 130, 150, and 170, respectively, which is formed over gate oxide layer 128, 148, and 168, respectively. In addition, oxide spacers 132, 152, and 172 are formed along the sidewalls of gates 130, 150, and 170, respectively.

In accordance with the grandparent invention, NMOS transistor 160 is formed to have a physical gate length L1, which is equal to the minimum physical gate length, while transistor 140 is formed to have a physical gate length L2 which is smaller than length L1.

Conventionally, the design rule or process technology that is used to form MOS transistors defines a minimum physical gate length. The minimum physical gate length is not the smallest feature size that is photolithographically obtainable, but is the smallest feature size that is allowed under the rule for CMOS devices.

Thus, for example, a 0.35 micron design rule or process technology defines a minimum physical gate length of 0.35 microns for the NMOS transistors. Similarly, a 0.25 micron design rule or process defines a minimum physical gate length of 0.25 microns for the NMOS transistors. As a result, the minimum physical gate length is commonly used to identify the design rule or process technology that is used to form MOS transistors.

FIG. 3 shows a graph that illustrates a series of programming voltages versus physical floating gate lengths in accordance with the grandparent invention. As shown in FIG. 3, with a physical floating gate length of 0.25 microns based on a 0.35 micron design rule or technology (and conventional doping concentrations for the well, source, and drain), holes flowing from source region 142 to drain region 144 become hot when drain 144 is grounded and the voltage applied to n-well 114 and source region 142 is equal to approximately 3.5 volts.

CMOS devices fabricated with a 0.35 micron design rule conventionally use a power supply which has an average voltage of 3.3 volts and a maximum voltage of approximately 3.6 volts. Thus, by shortening the physical gate length of memory transistor 140 (which also shortens the channel length) with respect to the design rule or technology, transistor 140 can be programmed with the existing CMOS power supply.

Further, the maximum voltage rating, which is the maximum voltage that can be applied for a short period of time without destroying the device, is approximately 4.6 volts with a 0.35 micron design rule. Thus, if 4.5 volts can be obtained from an external or other power supply, memory transistor 140 can be formed with a physical gate length which is only slightly less than the design rule, e.g., approximately 0.30 microns.

Similarly, as extrapolated from FIG. 3, with a physical gate length of 0.15 microns based on a 0.25 micron design rule or technology (and conventional doping concentrations for the well, source, and drain), holes flowing from source region 142 to drain region 144 become hot when the voltage applied to well 114 and source region 142 is equal to approximately 2.7 volts.

CMOS devices fabricated with a 0.25 micron design rule conventionally use a power supply which has an average voltage of 2.5 volts, a maximum voltage of approximately 2.75 volts, and a maximum voltage rating in excess of 3.25 volts. Thus, by shortening the physical gate length with respect to the design rule, memory transistor 140 can be programmed by voltages that do not exceed the maximum voltage rating for the design rule.

Further, the Frohmann-Bentchkowsky EPROM cell of the grandparent invention is integratable into a standard CMOS fabrication process without any additional process steps. The cell of the grandparent invention primarily differs from a conventional CMOS transistor in that the physical gate length is shorter (and the gate is electrically isolated).

FIG. 4 shows a schematic diagram that illustrates a portion of a memory device 200 in accordance with the parent invention. As shown in FIG. 4, device 200 includes a plurality of memory cells 210 which are arranged in rows and columns.

Each memory cell 210, in turn, includes a memory transistor 212 which has a source, a drain, and a floating gate, and an access transistor 214 which has a source connected to ground, a drain connected to the drain of the memory transistor, and a gate.

Each memory transistor 212 is a p-channel Frohmann-Bentchkowsky transistor which is formed in accordance with the grandparent invention, i.e., formed to have a physical gate length that allows programming via hot punchthrough holes without breaking down the junction, while each access transistor 214 is an n-channel CMOS device.

As further shown in FIG. 4, device 200 also includes a plurality of page lines PL1-PLm that are formed adjacent to the rows of cells 210 so that each page line PL is connected to the source of each memory transistor 212 in a row of cells 210.

In addition, device 200 further includes a plurality of enable lines EL1-ELn, and a plurality of sense lines SL1-SLn. The enable lines EL1-ELn are formed adjacent to the columns of cells 210 so that each enable line EL is connected to the gate of each access transistor 214 in a column of cells 210.

Like the enable lines EL1-ELn, the sense lines SL1-SLn are also formed adjacent to the columns of cells 210. However, unlike the enable lines EL1-ELn, each sense line SL is connected to the drain of each memory transistor 212 in a column of cells 210.

As additionally shown in FIG. 4, device 200 also includes a plurality of isolation transistors IT1-ITn, and a plurality of sense amps SA1-SAn. The isolation transistors IT1-ITn are n-channel CMOS devices which are connected to the sense lines SL1-SLn so that each isolation transistor IT is connected to one of the sense lines SL. Further, an isolation line IL is connected to the gate of each isolation transistor IT.

The sense amps SA1-SAn, in turn, are connected to the isolation transistors IT1-ITn so that each sense amp SA is connected to one of the isolation transistors IT.

In operation, device 200 is programmed row by row (page by page) by applying ground to the isolation line IL. Grounding the isolation line IL turns off the isolation transistors IT1-ITn which, in turn, isolates the sense amps SA1-SAn from the memory cells 210.

In addition, a programming voltage, such as 3.5 volts (based on a 0.35 micron design rule), is applied to the page line PL which is connected to the row (or page) of cells 210 that contains the to-be-programmed cell(s), while ground is connected to the remainder of the page lines PL.

Further, a first enable voltage, such as 3.3 volts (based on a 0.35 micron design rule), is applied to each enable line EL1-ELn that is connected to a column of cells 210 that contains a to-be-programmed cell 210, while ground is connected to the remainder of the enable lines EL1-ELn.

For example, if only cell A in FIG. 4 is to be programmed, the programming voltage is applied to page line PL1, while ground is applied to page lines PL2-PLm. In addition, the first enable voltage is applied to enable line EL1, while ground is applied to enable lines EL2-ELn. (Isolation line IL is also grounded). When these bias conditions are applied, cell A in FIG. 4 is programmed with hot punchthrough holes as described with respect to the grandparent invention.

Device 200 is read row by row (page by page) by applying a second enable voltage, such as 3.3 volts (based on a 0.35 micron design rule), to the isolation line IL. Applying the second enable voltage to the isolation line IL turns on the isolation transistors IT1-ITn which, in turn, connects the sense amps SA1-SAn to the memory cells 210.

In addition, a read voltage, such as 2.0 volts (based on a 0.35 micron design rule), is applied to the page line PL which is connected to the row (or page) of cells 210 that contains the to-be-read cells, while ground is connected to the remainder of the page lines PL. Further, ground is applied to each enable line EL1-ELn.

For example, if cells A, B, and C in FIG. 4 are to be read, the read voltage is applied to page line PL1, while ground is applied to page lines PL2-PLM. In addition, ground is applied to each enable line EL1-ELn, while the second enable voltage is applied to the isolation line IL.

When these bias conditions are applied, a current flows to the sense amps SA1-SAn when the cells have been programmed, while little or no current flows when the cells have not been programmed. Each sense amp SA then detects the difference in current magnitude to determine if a logic zero or a logic one is stored in the cell.

FIG. 5 shows a plan view that illustrates a layout 300 of a portion of device 200 in accordance with the parent invention. FIG. 6 shows a cross-sectional view taken along line 6--6 of FIG. 5.

As shown in FIGS. 5 and 6, the n-channel access transistors 214, which each have a source 220, a drain 222, and a gate 224, are formed in a p-type substrate 226, while the memory transistors 212, which each have a source 230, a drain, 232, and a floating gate 234, are formed in n-wells 236 which, in turn, are formed in p-substrate 226. (The access transistors 214 may alternately be formed in a p-well).

As further shown in FIGS. 5 and 6, two memory transistors 212 are formed in each n-well 236 so that the sources 230 of each memory transistor 212 share a page line contact 240 which is connected to a page line PL. In addition, each n-well 236 has a well contact 242 that is also connected to the page line PL. (Each of the memory transistors 212 in a row may alternately be formed in a single n-well).

The drains 232 of each memory transistor 212 in a column are electrically connected together, and to the drains 222 of each access transistor 214 in the column via drain contacts 244 and a sense line SL.

In addition, the source 220 of each access transistor 214 in a column is connected to the source 220 of another access transistor 214 in the column, and is electrically connected to the sources 220 of each access transistor 214 in the column via ground contacts 246. The sources 220 of each access transistor 214 in a row are connected together via a buried source line 250. Further, the gates 224 of the access transistors 214 in a column are electrically connected together via enable contacts 252 and an enable line EL.

FIG. 7 shows a schematic diagram that illustrates a portion of a device 400 in accordance with a first alternate embodiment of the parent invention. As shown in FIG. 7, device 400 is similar to device 200 of FIG. 4 and, as a result, uses the same reference numerals to designate the structures which are common to both devices.

As further shown in FIG. 7, device 400 primarily differs from device 200 in that a plurality of multiplexors MUX1-MUXn are used in lieu of the isolation transistors IT1-ITn. Each multiplexor MUX, in turn, has an input connected to ground, an input connected to a sense amp SA that corresponds with a column of cells 210, and an output connected to the source of each access transistor 214 in the column of cells 210 (rather than the drain as was the case with the isolation transistors IT).

In operation, device 400 is programmed row by row (page by page) by applying ground to the isolation line IL. Grounding the isolation line IL causes the multiplexors MUXl-MUXn to connect the sense lines SL1-SLn to ground.

In addition, the programming voltage is applied to the page line PL which is connected to the row (or page) of cells 210 that contains the to-be-programmed cell(s), while ground is connected to the remainder of the page lines PL.

Further, the first enable voltage is applied to each enable line EL1-ELn that is connected to a column of cells 210 that contains a to-be-programmed cell 210, while ground is connected to the remainder of the enable lines EL1-ELn.

For example, if only cell A in FIG. 7 is to be programmed, the programming voltage is applied to page line PLl, while ground is applied to page lines PL2-PLM. In addition, the first enable voltage is applied to enable line EL1, while ground is applied to enable lines EL2-ELn. Isolation line IL is also grounded. When these bias conditions are applied, cell A in FIG. 7 is programmed with hot punchthrough holes as described with respect to the grandparent invention.

Device 400 is read row by row (page by page) by applying the second enable voltage to the isolation line IL. Applying the second enable voltage to the isolation line IL causes the multiplexors MUX1-MUXn to connect the sense amps SA1-SAn to the sense lines SL1-SLn.

In addition, the read voltage is applied to the page line PL which is connected to the row (or page) of to-be-read cells, while ground is connected to the remainder of the page lines PL. Further, the first enable voltage is applied to each enable line EL1-ELn.

For example, if cells A, B, and C in FIG. 7 are to be read, the read voltage is applied to page line PL1, while ground is applied to page lines PL2-PLm. In addition, the first enable voltage is also applied to each enable line EL1-ELn. The second enable voltage is also applied to the isolation line IL. When these bias conditions are applied, a current flows to the sense amps SA1-SAn when the cells have been programmed, while little or no current flows when the cells have not been programmed.

FIG. 8 shows a plan view that illustrates a layout 500 of a portion of device 400 in accordance with the parent invention. FIG. 9 shows a cross-sectional view taken along line 9--9 of FIG. 8.

As shown in FIGS. 8 and 9, layout 500 is similar to layout 300 of FIG. 5 and, as a result, utilizes the same reference numerals to identify the structures which are common to both layouts.

Layout 500 principally differs from layout 300 in that the drain 232 of a memory transistor 212 in a column is only connected to the drain 222 of one access transistor 214 in the column, and one access transistor 214 is formed between adjacent pairs of page lines PL. Further, each of the memory transistors 212 in a row are formed in a single n-well 236. (Each pair of memory transistors 212 in a row may alternately be formed in a single n-well).

In addition, the sources 220 of each access transistor 214 in a column of cells is connected to all of the sources 220 of the access transistors 214 in the column via a source contact 260 and a sense line SL.

FIG. 10 shows a schematic diagram that illustrates a portion of a device 1000 in accordance with a second alternate embodiment of the parent invention. FIGS. 11A-11E and 12A-12E are timing diagrams that illustrate the operation of device 1000. As shown in FIG. 10, device 1000 is similar to device 400 of FIG. 7 and, as a result, uses the same reference numerals to designate the structures which are common to both devices.

As further shown in FIG. 10, device 1000 differs from device 400 in that a plurality of enable lines EL1-ELm are formed adjacent to the rows of cells 210 in device 1000 rather than adjacent to the columns of cells 210 as in device 400.

In addition, sense amps SA1-SAn are connected to sense lines SL1-SLn in device 1000 rather than being isolated from sense lines SL1-SLn as in device 400. As shown, each sense amp is formed from two inverters such that the output of a first inverter is connected to a sense line SL and the input of a second inverter, while the output of the second inverter is connected to one of a plurality of inverted sense lines IV1-IVn and the input of the first inverter. Further, an amp line AL is utilized to turn on and off sense amps SA1-SAn.

Device 1000 also includes a plurality of n-channel precharge transistors 1010 which are connected to the sense lines SL1-SLn and the inverse sense lines IV1-IVn so that a precharge transistor 1010 is connected to each of the sense lines SL and each of the inverse sense lines IV. Further, a precharge line PC is connected to the gate of each precharge transistor 1010.

Device 1000 additionally includes a plurality of isolation transistors 1020, and a series of input/output latches L1-Ln. Isolation transistors 1020 are n-channel CMOS devices which are connected between the sense lines SL1-SLn and the latches L1-Ln, and between the inverse sense lines IV1-IVn and the latches L1-Ln. Further, an isolation line ISL is connected to the gate of each isolation transistor 1020.

In operation, device 1000 is programmed row by row (page by page) by first clocking a logic low onto each sense line SL that is connected to a column of cells 210 that contains a to-be-programmed cell 210, while a logic high is clocked onto each sense line SL that is connected to a column of cells 210 that contains a not-to-be-programmed cell 210.

As shown in FIGS. 11A-11E, logic lows and highs, which are output by latches L1-Ln, are clocked onto the sense lines SL1-SLn by first raising the voltages on the amp line AL and the isolation line ISL at the beginning of a write cycle. Once the lows and highs are on the sense lines SL1-SLn, the voltage on the isolation line ISL is lowered.

After the voltage on the isolation line ISL has been lowered, the programming voltage on the page line PL which is connected to the row (or page) of cells 210 that contains the to-be-programmed cell(s) is raised, while ground is connected to the remainder of the page lines PL.

Further, the first enable voltage on the enable line EL that is connected to the row of cells 210 that are to be programmed is raised after the voltage on the page line PL has been raised, while ground is connected to the remainder of the enable lines EL. In addition, precharge line PC is also grounded.

For example, if only cell A in FIG. 10 is to be programmed, once the lows and highs have been clocked onto the sense lines SL1-SLn, the programming voltage is applied to page line PL1, while ground is applied to page lines PL2-PLM. In addition, the first enable voltage is applied to enable line EL1, while ground is applied to enable lines EL2-ELn. Precharge line PC is also grounded. When these bias conditions are applied, cell A in FIG. 10 is programmed with hot punchthrough holes as described with respect to the grandparent invention.

As shown in FIG. 12A-12E, device 1000 is read row by row (page by page) by first raising the voltage on the precharge line PC and the amp line AL at the beginning of a read cycle. Raising the voltage on the precharge line PC turns on the precharge transistors 1010 which, in turn, places approximately VCC/2 on each sense line SL and each inverse sense line IV1-IVn.

Following this, the voltage on the precharge line PC is again lowered to turn off precharge transistors 1010. Next, the read voltage on the page line PL which is connected to the row (or page) of to-be-read cells is raised, while ground is connected to the remainder of the page lines PL.

Further, the first enable voltage on the enable line EL which is connected to the row (or page) of to-be-read cells is raised, while ground is connected to the remainder of the enable lines EL.

For example, if cells A, B, and C in FIG. 10 are to be read, the read voltage is applied to page line PL1, while ground is applied to page lines PL2-PLm. In addition, the first enable voltage is also applied to enable line EL1, while ground is applied to enable lines EL2-ELm.

When these bias conditions are applied, if cells A and C are programmed, then cells A and C will conduct a current which, in turn, raises the voltages on sense lines SL1 and SLn. The imbalance between sense line SL1 and inverse sense line IV1; and between SLn and IVn, will cause sense amps SA1 and SAn to flip.

This is similar to the approach used in DRAMs. The inverse sense lines IV1-IVn can be connected to the sense lines of a second memory block which, in turn, allows the shared use of the sense amps between the two blocks.

FIG. 13 shows a schematic diagram that illustrates a portion of a device 1300 in accordance with a third alternate embodiment of the parent invention. As shown in FIG. 13, device 1300 is similar to device 400 of FIG. 7 and, as a result, uses the same reference numerals to designate the structures which are common to both devices.

As further shown in FIG. 13, device 1300 differs from device 400 in that the multiplexors MUX1-MUXn have been eliminated by using the sense amp circuitry disclosed in FIG. 10.

Device 1300 operates the same as device 400 except that device 1300 clocks logic lows onto the sense lines SL that are to be programmed rather than muxing ground onto the sense lines SL as done with device 400.

FIG. 14 shows a schematic diagram that illustrates a Frohmann-Bentchkowsky EPROM memory cell 1400 in accordance with the present invention. Cell 1400 is similar to the memory cells 210 of device 200 shown in FIG. 4 and, as a result, utilizes the same reference numerals to designate the structures which are common to both cells.

As with cell 210, memory cell 1400 includes a memory transistor 212 which has a source, a drain, and a floating gate, and an access transistor 214 which has a source connected to ground, a drain connected to the drain of memory transistor 212, and a gate connected to receive an enable voltage V_(E).

In addition, as with cell 210, memory transistor 212 is a p-channel Frohmann-Bentchkowsky transistor which is formed in accordance with the grandparent invention, i.e., formed to have a physical gate length that allows programming via hot punchthrough holes without breaking down the junction, while access transistor 214 is an n-channel CMOS device.

Memory cell 1400 differs from memory cell 210 in that memory cell 1400 includes a n-channel, diode-connected transistor 1410 which functions as a non-linear circuit element. Transistor 1410 has a gate and drain connected to the drains of transistors 212 and 214 at an output node N_(O), and a source connected to ground.

Transistors 214 and 1410 are formed to have a physical gate length which is greater than the physical gate length of transistor 212. For example, transistors 214 and 1410 can have a physical gate length that is equal to the minimum physical gate length allowed by the design rule, while transistor 212 is formed to have a physical gate length that is less than the minimum physical gate length.

In operation, cell 1400 is programmed by applying a programming voltage Vdd/Vpp, such as 3.5 volts (0.35 micron design rule), to the source of memory transistor 212, while the enable voltage V_(E), such as 3.3 volts (0.35 micron design rule), is applied to the gate of transistor 214 to turn on transistor 214. When these bias conditions are applied, cell 1400 is programmed with hot electrons generated by punchthrough holes as described with respect to the grandparent invention.

Cell 1400 is read by applying a read voltage, such as 1.0-2.0 volts (0.35 micron design rule), to the source of memory transistor 212, while ground is applied to the gate of transistor 214 to turn off transistor 214.

When memory transistor 212 is programmed, transistor 212 provides a low resistance current path. As a result, very little voltage is dropped across memory transistor 212. A voltage approximately equal to the read voltage at the gate and drain of transistor 1410 turns on transistor 1410 which allows a current to flow through transistors 212 and 1410. Thus, when cell 1400 is programmed, a first voltage approximately equal to the read voltage is present on output node N_(O).

When memory transistor 212 is not programmed, transistor 212 is nonconductive (except for a leakage current). In accordance with the present invention, transistor 1410 is formed to be nonconductive (turned off) when transistor 212 is nonconductive (turned off).

When transistors 212, 214, and 1410 are each turned off, the transistors form an equivalent capacitive network which limits the voltage across transistor 212 to less than one-half of the read voltage (the capacitances of transistors 214 and 1410 are in parallel and therefore are combined).

Thus, when transistor 212 is not programmed (and transistors 214 and 1410 are turned off), a second voltage which is less than the read voltage, but greater than one-half of the read voltage is present on output node N_(O).

To insure that transistor 1410 remains nonconductive when transistor 212 is nonconductive, the threshold voltage of transistor 1410 is set to a voltage which is less than the first voltage and greater than the second voltage so that no appreciable current will flow through transistor 1410 when transistor 212 is nonconductive. (Although the threshold voltage of transistor 1410 can be set to be less than the second voltage, this forces a larger voltage across memory transistor 212.) The logic state stored in cell 1400 is then determined by sensing the voltage on output node N_(O). One method for sensing the voltage on output node N_(O) is to use an inverter which has a trip voltage which is less than the first voltage and greater than the second voltage.

FIG. 15 shows a schematic diagram that illustrates a portion of a memory device 1500 in accordance with the present invention. Device 1500 is similar to cell 1400 and, as a result, utilizes the same reference numerals to designate the common structures.

As shown in FIG. 15, device 1500 differs from cell 1400 in that device 1500 also includes an inverter 1510 which is connected to output node N_(O) to sense the voltage on output node N_(O). Inverter 1510 outputs, for example, a logic zero when the voltage on output node N_(O) is greater than the trip voltage, and a logic one when the voltage on output node N_(O) is less than the trip voltage.

One of the advantages of cell 1400 is that cell 1400 can withstand large read voltages, such as 3.3 to 5 volts (0.35 micron design rule) without inadvertently being programmed. In the present invention, when memory transistor 212 is not programmed, a voltage less than one-half of the read voltage is dropped across transistor 212.

Thus, when a 5 volt read voltage is used, a voltage less than 2.5 volts is dropped across transistor 212 which, in turn, is less than the approximately 3.5 volts (0.35 micron design rule) that is applied across memory transistor 212 when transistor 212 is programmed.

Without transistor 1410, the logic state of the resulting cell can be read by turning off transistor 214 and sensing the magnitude of the current that flows out of transistor 212 as discussed above with respect to device 200 of FIG. 4.

Alternately, the logic state of a cell without transistor 1410 can be read by turning on transistor 214 and sensing the voltage on output node N_(O). When the cell without transistor 1410 is programmed and transistor 214 is turned on, the voltage on the output node N_(O) is approximately equal to the read voltage.

When a cell without transistor 1410 is not programmed and transistor 214 is turned on, the voltage on the output node N_(O) is near ground. Although discrimination of the logic state stored in the cell is easy when the output voltage N_(O) is either near the read voltage or near ground, nearly all of the read voltage is dropped across memory transistor 212 when the cell is not programmed.

When read voltages in the range of 1.0-2.0 volts are used, problems typically do not arise. However, when read voltages in the range of 3.3-5.0 volts (0.35 micron design rule) are used (such as in a trim-bit circuit), these same voltages are dropped across memory transistor 212 which, in turn, can lead to transistor 212 being inadvertently programmed. Thus, by utilizing transistor 1410, the present invention prevents inadvertent programming when high read voltages are used.

FIG. 16 shows a schematic diagram that illustrates a portion of a memory device 1600 in accordance with the present invention. Device 1600 is similar to device 200 and, as a result, utilizes the same reference numerals to designate the structures which are common to both device.

As with device 200, each page line PL1-PLm is connected to the sources of each memory transistor 212 in a row of cells 1400, while each enable line EL1-ELn is connected to the gates of each access transistor 214 in a column of cells 1400. In addition, each sense line SL1-SLn is connected to the drains of each memory transistor 212 in a column of cells 1400.

As shown in FIG. 16, device 1600 differs from device 200 in that device 1600 utilizes memory cells 1400 rather than memory cells 210. Device 1600 also utilizes a series of inverters IV1-IVn in lieu of the sense amps SA1-SAm and isolation transistors IT1-ITn. Device 1600 is programmed and read in the same manner that device 200 is programmed and read (except that voltages are not applied to isolation line IL of FIG. 4).

FIG. 17 shows a schematic diagram that illustrates a portion of a memory device 1700 in accordance with the present invention. Device 1700 is similar to device 200 and, as a result, utilizes the same reference numerals to designate the structures which are common to both device.

As shown in FIG. 17, device 1700 differs from device 200 in that device 1700 includes a plurality of n-channel, diode-connected transistors 1710 which are connected to the sense lines SL1-SLn so that a transistor 1710 is connected to each sense line SL. Transistor 1710 is the same as transistor 1410.

Device 1700 also utilizes a series of inverters IV1-IVn in lieu of the sense amps SA1-SAm and isolation transistors IT1-ITn. Device 1700 is programmed and read in the same manner that device 200 is programmed and read (except that voltages are not applied to isolation line IL of FIG. 4).

FIG. 18 shows a schematic diagram that illustrates a Frohmann-Bentchkowsky EPROM memory cell 1800 in accordance with the present invention. Cell 1800 is similar to the memory cell 1400 and, as a result, utilizes the same reference numerals to designate the structures which are common to both cells.

Memory cell 1800 differs from memory cell 1400 in that memory cell 1800 also includes a n-channel transistor 1810 which is connected in series with transistor 1410. Transistor 1810, which is always on when cell 1800 is read, and always off when cell 1800 is programmed, functions as a voltage reducer that further reduces the voltage drop across memory transistor 212 by raising the voltage on the source of transistor 1410 above zero.

In addition, as with cell 1400, an inverter can be connected to the output node N_(O) of cell 1800 to sense the voltage on the output node N_(O). Further, cells 1800 can be used in lieu of cells 1400 in memory device 1600, which is operated in the same manner. In addition, transistor 1810 can be connected between each transistor 1710 and ground in memory device 1700.

FIG. 19 shows a schematic diagram that illustrates a Frohmann-Bentchkowsky EPROM memory cell 1900 in accordance with the present invention. Cell 1900 is similar to the memory cell 1400 and, as a result, utilizes the same reference numerals to designate the structures which are common to both cells.

Memory cell 1900 differs from memory cell 1400 in that memory cell 1900 utilizes a p-channel transistor 1910 in lieu of n-channel transistor 1410. (Transistor 1910 can be formed in the n-well that includes the p-channel memory transistor 212.)

In operation, the threshold voltage of transistor 1910 is set to a voltage which insures that transistor 1910 is conductive (turned on) when memory transistor 212 is programmed (when the voltage on the output node N_(O) is near the read voltage), and nonconductive (turned off) when memory transistor 212 is not programmed (when the voltage on output node N_(O) is less than the read voltage and greater than one-half the read voltage).

In addition, as with cell 1400, an inverter can be connected to the output node N_(O) of cell 1900 to sense the voltage on the output node N_(O). Further, cells 1900 can be used in lieu of cells 1400 in memory device 1600, which is operated in the same manner. In addition, transistor 1910 can be used in lieu of transistor 1710 in memory device 1700.

FIG. 20 shows a schematic diagram that illustrates a Frohmann-Bentchkowsky EPROM memory cell 2000 in accordance with the present invention. Cell 2000 is similar to the memory cell 1900 and, as a result, utilizes the same reference numerals to designate the structures which are common to both cells.

Memory cell 2000 differs from memory cell 1900 in that memory cell 2000 also includes a n-channel transistor 2010 which is connected in series with transistor 1910. Transistor 2010, which is always on when cell 2000 is read, and always off when cell 2000 is programmed, further reduces the voltage drop across memory transistor 212 by raising the voltage on the drain of transistor 1910 above zero.

In addition, as with cell 1400, an inverter can be connected to the output node N_(O) of cell 2000 to sense the voltage on the output node N_(O). Further, cells 2000 can be used in lieu of cells 1400 in memory device 1600, which is operated in the same manner. In addition, transistors 1910 and 2010 can be used in lieu of transistor 1710 in memory device 1700.

FIG. 21 shows a schematic diagram that illustrates a memory circuit 2100 in accordance with the present invention. As shown in FIG. 21, circuit 2100 includes a comparator 2110 and a pair of memory cells 1800 which are connected to comparator 2110. In operation, one of the two cells 1800 is designated as a reference cell and is either permanently programmed or not programmed. The voltage from the other cell 1800 is then compared to the reference voltage by comparator 2110.

It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby. 

What is claimed is:
 1. A memory cell comprising:a memory transistor of a first conductivity type having a source, a drain, and a floating gate, the memory transistor having a physical gate length; a first transistor of a second conductivity type having a source, a drain connected to the drain of the memory transistor, and a gate, the first transistor having a physical gate length defined by a design rule which is greater than the physical gate length of the memory transistor; and a non-linear circuit element connected to the drains of the first transistor and the memory transistor.
 2. The memory cell of claim 1 and further comprising an inverter having an input connected to the drain of the memory transistor.
 3. The memory cell of claim 2 and further comprising a second transistor of the second conductivity type having a source, a drain connected to the non-linear circuit element, and a gate, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 4. The memory cell of claim 1 wherein the non-linear circuit element includes a second transistor of the second conductivity type having a gate connected to the drains of the first transistor and the memory transistor, a source, and a drain, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 5. The memory cell of claim 4 wherein the physical gate length of the memory transistor is less than a minimum gate length defined by the design rule.
 6. The memory cell of claim 4 and further comprising an inverter having an input connected to the drain of the memory transistor.
 7. The memory cell of claim 6 and further comprising a third transistor of the second conductivity type having a source, a drain connected to the source of the second transistor, and a gate, the third transistor having a physical gate length greater than the physical gate length of the memory transistor.
 8. The cell of claim 1 wherein the non-linear circuit element includes a second transistor of the first conductivity type having a source connected to the drains of the first transistor and the memory transistor, a drain, and a gate, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 9. A memory circuit comprising:a pair of memory cells, each cell having:a memory transistor of a first conductivity type having a source, a drain, and a floating gate, the memory transistor having a physical gate length; a first transistor of a second conductivity type having a source, a drain connected to the drain of the memory transistor, and a gate, the first transistor having a physical gate length greater than the physical gate length of the memory transistor; and a second transistor of the second conductivity type having a gate connected to the drains of the first transistor and the memory transistor, a source, and a drain, the second transistor having a physical gate length greater than the physical gate length of the memory transistor; and a comparator connected to the drains of the memory transistors of each memory cell.
 10. A method for operating a memory cell, the memory cell having:a memory transistor of a first conductivity type having a source, a drain, and a floating gate, the memory transistor having a physical gate length; a first transistor of a second conductivity type having a source, a drain connected to the drain of the memory transistor, and a gate, the first transistor having a physical gate length defined by a design rule which is greater than the physical gate length of the memory transistor; and a non-linear circuit element connected to the drains of the first transistor and the memory transistor, the method comprising the programming steps of:applying a first voltage to the source of the memory transistor; and applying a second voltage to the gate of the first transistor sufficient to turn on the first transistor; the method further comprising the read steps of:applying the first voltage to the source of the memory transistor; and applying a third voltage to the gate of the first transistor sufficient to turn off the first transistor.
 11. The method of claim 10 wherein the non-linear circuit element includes a second transistor of the second conductivity type having a gate connected to the drains of the first transistor and the memory transistor, a source, and a drain, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 12. The method of claim 11 and further comprising an inverter having an input connected to the drain of the memory transistor.
 13. The method of claim 12 and further comprising a third transistor of the second conductivity type having a source, a drain connected to the source of the second transistor, and a gate, the third transistor having a physical gate length greater than the physical gate length of the memory transistor, the third transistor always being on during a read and always off during programming.
 14. The method of claim 11 wherein the physical gate length of the memory transistor is less than a minimum gate length defined by the design rule.
 15. A memory device comprising:a row of memory cells, each memory cell including:a memory transistor of a first conductivity type having a source, a drain, and a floating gate, the memory transistor having a physical gate length; and a first transistor of a second conductivity type having a source, a drain connected to the drain of the memory transistor, and a gate, the first transistor having a physical gate length defined by a design rule which is greater than the physical gate length of the memory transistor; a page line connected to the source of each memory transistor in the row; a plurality of enable lines formed so that each enable line is connected to the gate of a first transistor; a plurality of sense lines formed so that each sense line is connected to each memory cell in a column of memory cells; and a plurality of non-linear circuit elements connected to the sense lines so that a non-linear circuit element is connected to each sense line.
 16. The memory device of claim 15 wherein the non-linear circuit element includes a second transistor of the second conductivity type having a gate connected to the drains of the first transistor and the memory transistor, a source, and a drain, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 17. The memory device of claim 16 wherein each memory cell further includes a third transistor of the second conductivity type having a source, a drain connected to the source of the second transistor, and a gate, the third transistor having a physical gate length greater than the physical gate length of the memory transistor.
 18. The memory device of claim 16 wherein the physical gate length of the memory transistor is less than a minimum gate length defined by the design rule.
 19. A method for operating a memory device, the memory device comprising:a row of memory cells, each memory cell including:a memory transistor of a first conductivity type having a source, a drain, and a floating gate, the memory transistor having a physical gate length; a first transistor of a second conductivity type having a source, a drain connected to the drain of the memory transistor, and a gate, the first transistor having a physical gate length greater than the physical gate length of the memory transistor; and a non-linear circuit element connected to the drains of the first transistor and the memory transistor; a page line connected to the source of each memory transistor in the row; a plurality of enable lines formed so that each enable line is connected to the gate of a first transistor; and a plurality of sense lines formed so that each sense line is connected to each memory cell in a column of memory cells, the method comprising the programming steps of:applying a first voltage to the page line; and applying a second voltage to the gate of each first transistor in a to-be-programmed cell sufficient to turn on each first transistor in a to-be-programmed cell; the method further comprising the read steps of:applying the first voltage to the page line; and applying a third voltage to the gate of each first transistor sufficient to turn off each first transistor.
 20. The method of claim 19 wherein the non-linear circuit element includes a second transistor of the second conductivity type having a gate connected to the drains of the first transistor and the memory transistor, a source, and a drain, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 21. A memory device comprising:a row of memory cells, each memory cell including:a memory transistor of a first conductivity type having a source, a drain, and a floating gate, the memory transistor having a physical gate length; a first transistor of a second conductivity type having a source, a drain connected to the drain of the memory transistor, and a gate, the first transistor having a physical gate length defined by a design rule which is greater than the physical gate length of the memory transistor; and a non-linear circuit element connected to the drains of the first transistor and the memory transistor; a page line connected to the source of each memory transistor in the row; a plurality of enable lines formed so that each enable line is connected to the gate of a first transistor; and a plurality of sense lines formed so that each sense line is connected to each memory cell in a column of memory cells.
 22. The memory device of claim 21 wherein the non-linear circuit element includes a second transistor of the second conductivity type having a gate connected to the sense line, a source, and a drain, the second transistor having a physical gate length greater than the physical gate length of the memory transistor.
 23. The memory device of claim 21 wherein the physical gate length of the memory transistor is less than a minimum gate length defined by the design rule.
 24. The memory device of claim 21 and further including a plurality of third transistors of the second conductivity type having a drain connected to the source of the second transistor, a source, and a gate, the third transistor having a physical gate length greater than the physical gate length of the memory transistor. 